To produce ultraviolet (UV) laser light having medium power, e.g., in a power range from about 0 to 30 Watt, from a first (i.e., fundamental) wavelength (e.g., λ1=1064 nm) delivered by a laser oscillator, it is possible, in a first step, to generate, using a first nonlinear crystal with noncritical phase matching, a frequency-doubled laser beam having a second (i.e., second harmonic) wavelength (e.g., λ2=λ1/2=532 nm) in a process called “second harmonic generation” (SHG). Here, the beam having the fundamental wavelength and the beam having the second harmonic are linearly polarized and have polarization directions perpendicular to each other. Using a second nonlinear crystal with critical phase matching, the fundamental wave and the second harmonic then generate a third laser beam having a sum-frequency according to 1/λ3=1/λ1+1/λ2=3/λ1 in a process called “sum-frequency generation” (SFG). In the present example, the third laser beam would have a wavelength λ3=355 nm, which lies in the UV range.
In SFG, the critical phase matching in the nonlinear crystal causes the wave vectors, k, of the three waves participating in the frequency mixing to fulfill the condition k3=k2+k1. For SHG the wave vectors follow the expression k2=2 k1. Because of the birefringent properties of the nonlinear crystal, the critical phase matching in SFG leads, however, to a first beam of the two incoming laser beams (e.g., extraordinary polarized wave) running away from the second incoming laser beam (e.g., ordinarily polarized wave) at a so-called “walk-off” angle. The two laser beams are then, after a certain propagation distance, separated within the nonlinear crystal and have what is called a spatial walk-off.
In the generation of UV laser light of medium power in the manner described initially, the fundamental wave in the second nonlinear crystal is typically ordinary-polarized while the second harmonic in the nonlinear crystal is extraordinary-polarized, so that the walk-off effect occurs in the second nonlinear crystal. In the first nonlinear crystal, on the other hand, a noncritical phase matching is used, so that no walk-off effect occurs there, and the fundamental wave and the second harmonic emerge collinearly from the crystal.
The walk-off between the fundamental wave and the second harmonic in the second nonlinear crystal reduces the efficiency of conversion when generating the third harmonic (e.g., UV radiation), since the interaction length, in which the frequency conversion takes place, decreases. Compensating the walk-off effect extends the interaction length, the result being that the conversion efficiency when generating the UV radiation increases markedly.
DE10143709 A1 discloses a method for compensating the walk-off effect during frequency conversion. In this method a first nonlinear lithium triborate (LBO) crystal having noncritical phase matching is used for frequency doubling and a second LBO crystal with critical phase matching is used to generate the third harmonic. Between the first and the second nonlinear crystal there is arranged a birefringent crystal, in which nonlinear optical properties are avoided. The birefringent crystal generates a walk-off, which leads to a beam offset of the fundamental wave and the second harmonic on the second nonlinear crystal. The beam offset is directed opposite to the walk-off of the second LBO crystal and is intended to compensate this.
In the construction described in DE10143709 A1, however, the additional birefringent crystal has to be positioned in the beam path, leading to more adjustment work. Since the birefringence of the crystal that counteracts the walk-off effect of the second nonlinear crystal is temperature—dependent, the birefringent crystal used for separating the laser beams must also be maintained at a constant temperature.
EP 0503875 A2 discloses a further possibility for compensating walk-off in nonlinear crystals with critical phase matching. In this case, the walk-off between the ordinary and the extraordinary beam is compensated by both beams striking the nonlinear crystal with critical phase matching collinearly at an angle to the crystal surface. When the laser light strikes the crystal at an angle, refraction occurs and the wave vector of the radiation changes depending on the polarization direction by a different amount. With a correct choice of angle of incidence and the orientation of the crystal axis (axes) relative to the entrance face of the crystal it is therefore possible to compensate the walk-off effect. This solution requires a suitably cut and adjusted nonlinear crystal, however. The crystal axis/axes is/are typically not oriented perpendicular or parallel to the crystal surface, but instead run at an angle to the crystal surfaces. The angle at which the crystal is cut must therefore be extremely precise, since an imprecise cutting angle cannot be compensated by a tilting or rotation of the entire crystal.
US 2006/0250677 discloses a solution for compensating walk-off in which the laser beams are likewise incident on the nonlinear crystal at the same point, but have previously been separated by means of an optical system, so that the angles of incidence of the laser beams with respect to the crystal surface are different from each other. Whereas one of the laser beams strikes the crystal surface, e.g. perpendicularly with respect to the crystal surface, the other laser beam strikes the nonlinear crystal at non-perpendicular angle. A biprism is positioned in front of the nonlinear crystal as the optical system for separating the incident beams. When using laser beams of different wavelength, a dichroic beam splitter can optionally be used. However, since the crystal axis runs at an angle to the crystal surface, the cut of the nonlinear crystal should be extremely precise in order to achieve maximum walk-off compensation.